In the past, for example, in a known configuration, an extrusion press which uses copper, aluminum, or their alloys etc. to extrude tubular shaped products by double action extrusion comprises a cylinder platen and end platen which are arranged facing each other. The cylinder platen is provided with a main cylinder, main ram, pressure stem, and mandrel. The end platen is provided with a die. A container is provided between the pressure stem and the die so as to be able to advance and retract by a container cylinder.
The pressure stem, which has a dummy block arranged at its front end, has been attached through a main cross head to a main ram which is assembled in the main cylinder which is provided at the cylinder platen. At a center position of the pressure stem, a mandrel is arranged to be able to advance and retract accompanying the pressure stem together with the piercer cylinder rod. Further, the die has been attached to the end platen facing the pressure stem.
Between the pressure stem and the die, the container has been arranged to be able to advance and retract. In the container, a billet has been held. The pressure stem moves the billet which is held in the container to the die side whereby the billet is pushed and upset is completed. After upset, the mandrel advances and the billet is pierced. The mandrel stops at a predetermined advancing position in the die and the pressure stem again advances, whereby the billet is extruded as a tubular shaped product by double action extrusion.
In a double action of extrusion press, the front end part of the mandrel is made to stop at a predetermined position of the bearing part of the die to extrude the product. At that time, even if the relative positions of the mandrel and the bearing part of the die are pulled by the product, the mandrel position is held so that the stop position does not shift.
PLT 1 discloses a double action type of extrusion press where a piercer cylinder which is provided inside the main cylinder and a stop fastener which detaches from the axial center of the extrusion press to forcibly couple with the mandrel are provided. The stop fastener acts on a hydraulic pilot valve to start the supply of a certain amount of the pressurized oil medium to the rod side chamber of the piercer cylinder so as to hold a predetermined axial direction position (stop position) of the bearing part of the die. The supply of the pressurized oil medium is made to match the increase in volume of the piercer cylinder rod side chamber when the mandrel stops and the main ram advances during control to hold the position.
In this regard, the conventional type of double action extrusion press is designed to mechanically switch the hydraulic pilot valve through the stop fastener and connecting rod to supply a certain amount of pressurized oil medium to thereby hold the mandrel at a predetermined position of the bearing part of the die, so a delay occurs in control of exactly the stroke of movement corresponding to the land of the spool of the hydraulic pilot valve, and the front end part of the mandrel moves back and forth by several millimeters with respect to a predetermined stop position during extrusion.
For this reason, variation occurred in the thickness of the extruder tubular shaped product and a stable quality of tubular shaped product could not be obtained.
Furthermore, in a conventional double action extrusion press, there is the following problem:
When pushing a billet into the container by the pressure stem, then upsetting the billet and piercing the inside of the billet by the mandrel, then extruding the billet by a fixed mandrel, frictional force occurs between the billet and the mandrel surface and a pull force acts on the mandrel during extrusion. Due to this, the extrusion force which acts on the die decreases by that amount and the extrusion force could not be effectively utilized at the start of extrusion when the required extrusion force is most necessary.